The information about the illegitimate child comes from his service record. The lady was Elizabeth Younger Milroy of Rishton, the child born 8 Nov 1908 and the order for maintenance made in December 1908. The army record of it is all dated in March 1915 so whether he didn't pay up straight away - who knows.
The reason Ypres was discounted was because the entries on his record, which are quite clear, show service in UK, Egypt and the Mediterranean Expeditionary Force which at the time recorded was also Egypt. From these it was possible to cross check to battalions who served in those places.
BUT - I should have looked more closely at the service record and particularly at the medal records.
In March 1915 when the child paperwork was generated he was in 2/4th Battalion in Southport. He was with them later in May 1915 in Sussex. His medal record shows he served overseas with 1/4th Battalion and with 9th Battalion. The 9th were in Salonika and 1/4th were at Gallipoli and later Egypt BUT served in France from early 1917.
I'm sorry this is full of numbers but essentially, his written record does not include France. If he was with 1/4th after the move from Egypt then the possibility of being gassed in France/Flanders does exist. It is a bit peculiar that his discharge record shows him as A1 medical category which means that, at least according to the medics, if he had been gassed, it didn't affect him.
As to where he was gassed, that cannot be established. Given the mis-match between his record and his medal records we can't be sure where he was and when. There were five major engagements, Oct/Nov 16, Apr/May 15, Jul/Nov 17 (Passchendaele), Apr 18 (Lys) and Sep/Oct 18 which became called the 1st, 2nd etc battles of Ypres. Ypres was held by the allies from Oct 1914 and the salient was under constant attack by shell and gas outside the major engagements too.
In sum, his records are a mess so while one can guess at the sequence of his service, the next person may come up with another idea! Suffice to say that the family story of being gassed in France/Flanders does look a possibility - evidence though cannot be found - but who needs it!
MaxD